This is the second in a two-part series about coating failures on nonferrous metals and how they can be avoided. The first part of the article, which appeared in the October issue, listed common mistakes made in pretreatment and discussed pretreatment requirements for aluminum alloys. This article will explain conversion coatings and four-, five-, and six-stage washer systems.

Nonferrous metals are prone to post-oxidation and pitting under the powder film when they're rinsed in poor-quality tap water. Generally, levels of total dissolved solids [TDS] higher than 10 micromhos in final rinse water may create premature coating failure. In the pretreatment of aluminum, you must be aware of nonnegotiable standards. I mentioned these previously in part I of this article, but they bear repeating. You must accomplish the following as a minimum:

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